Palbociclib

• HR-Positive Breast Cancer
• Metastatic Breast Cancer
• Advanced Breast Cancer

Palbociclib is a selective cyclin-dependent kinase 4/6 (CDK4/6) inhibitor used in combination with endocrine therapy for hormone receptor-positive, HER2-negative advanced or metastatic breast cancer. It represents a significant advance in targeted therapy for breast cancer.

Mechanism of Action

Palbociclib inhibits CDK4 and CDK6, key regulators of cell cycle progression:

• CDK4/6 form complexes with cyclin D
• These complexes phosphorylate retinoblastoma protein (Rb)
• Phosphorylated Rb releases E2F transcription factors
• E2F activation promotes transition from G1 to S phase

By blocking CDK4/6, palbociclib:

• Prevents Rb phosphorylation
• Arrests cells in G1 phase
• Inhibits DNA synthesis and cell proliferation
• Works synergistically with endocrine therapy in HR+ breast cancer

Available Formulations

Palbociclib is available as oral capsules:

• 75 mg capsules
• 100 mg capsules
• 125 mg capsules

Take with food to ensure adequate absorption.

Medical Uses

FDA-Approved Indications:

• HR-positive, HER2-negative advanced or metastatic breast cancer in combination with:
  • An aromatase inhibitor as initial endocrine-based therapy in postmenopausal women, or
  • Fulvestrant in women with disease progression following endocrine therapy

Can be used in pre/perimenopausal women when combined with LHRH agonist.

Dosing Guidelines

Standard Dosing:

• 125 mg once daily for 21 days followed by 7 days off (28-day cycle)
• Continue until disease progression or unacceptable toxicity

Dose Modifications for Toxicity:

• First reduction: 100 mg once daily
• Second reduction: 75 mg once daily
• Discontinue if 75 mg not tolerated

Neutropenia Management:

• Hold for Grade 3-4 neutropenia until recovery to Grade ≤2
• Resume at same or reduced dose per guidelines

Important Safety Information

Warnings and Precautions:

• Neutropenia: Most common adverse reaction; monitor CBC before starting and at beginning of each cycle, on day 15 of first two cycles, and as clinically indicated
• Infections: Increased risk due to neutropenia
• Pulmonary embolism: Monitor for signs and symptoms
• Embryo-fetal toxicity: Can cause fetal harm

Contraindications:

• None specifically listed, but avoid in patients with severe hypersensitivity to palbociclib

Drug Interactions

Strong CYP3A4 Inhibitors (ketoconazole, itraconazole, clarithromycin, grapefruit):

• Avoid concurrent use
• If unavoidable, reduce palbociclib to 75 mg once daily

Strong CYP3A4 Inducers (rifampin, phenytoin, carbamazepine, St. John's Wort):

• Avoid concurrent use (will reduce palbociclib efficacy)

CYP3A4 Substrates with Narrow Therapeutic Index (cyclosporine, ergotamine):

• Monitor closely; palbociclib may increase their concentrations

Special Populations

• Hepatic Impairment:
  • Mild to moderate: No adjustment
  • Severe: 75 mg once daily
• Renal Impairment: No adjustment for mild to moderate; caution in severe
• Pregnancy: Avoid; use effective contraception during and for 3 weeks after
• Lactation: Avoid breastfeeding during and for 3 weeks after
• Males: Use effective contraception during and for 3 months after